1. Field of the Invention
The present invention relates to polymeric compounds which can be employed as multifunctional additives to lubricants.
2. Description of the Related Art
It is known that lubricating oils, and in particular crankcase oils intended for internal combustion engines, contain various additives used to improve the performance of the said oils when in use. Certain additives are employed to increase the viscosity index of the oil, while others ensure, for example, that the crankcase is kept clean by suspending insoluble deposits, also known as sludge, which form in the oil.
The additives intended to improve the viscosity index of the oil must have, on the one hand, a sufficient thickening effect on a light lubricating oil at high temperatures in order that the lubricating properties of this oil may correspond to that of a heavier lubricating oil and, on the other hand, a limited thickening effect on a light lubricating oil at low temperatures to avoid disturbing the characteristics of the said oil at these low temperatures. They are generally long-chain polymeric compounds such as, for example, polyisobutenes, polymethacrylates, polyalkylstyrenes, partially hydrogenated butadiene and styrene copolymers and amorphous ethylene and propene copolymers.
The additives ensuring that the crankcase is kept clean are, on the one hand, detergents such as, for example, polyvalent metal sulphonates, phenates or organic phosphates, which are particularly effective during "hot" running, and, on the other hand, dispersants such as, for example, succinimides or succinic esters alkylated with an isobutene or propene polymer on the carbon alpha to the carbonyl of the succinimide, these additives working more particularly during "cool" running, which is that of motor vehicles which stop frequently.
There have also been proposed additives for lubricating oils, intended to ensure simultaneously the improvement in the viscosity index of the oils and the dispersion of the sludge which they may contain. Such compounds are, for example, obtained by introducing nitrogenous monomers either by copolymerization or by grafting onto a thickening polymer. The nitrogenous dispersant monomers are preferably chosen from maleimides, vinylimidazolines, vinylpyrrolidones, vinylpyridines or N,N-dialkylaminoethyl methacrylates. The following patents claim the use of such compounds: EP 171,167, EP 167,195, EP 164,807, EP 145,369, U.S. Pat. No. 4,606,834, U.S. Pat. No. 3,732,334, BE 874,068.
Nevertheless, in lubricants, multifunctional additives exhibiting properties other than viscosity improvers and dispersants are sought after. In particular, formulators increasingly look for additives exhibiting a high thermal stability and a high oxidation resistance because of the increasing severity of engine operating conditions. An increase in the working temperature of the lubricant is also observed, particularly in diesel engines subjected to high loads, where the temperature at the top of the piston, in the vicinity of the first groove, may reach or even exceed 350.degree. C. Lubricants tend to oxidize when subjected to these high temperatures. This decomposition is proportional to the thermal stability and to the oxidation resistance of the oil and of the additives. It gives rise to the formation and to the deposition of resinous compounds and of carbonaceous substances on the piston walls. If these deposits become too great, they can initiate a gumming up or seizing of the piston and can increase the wear of the moving metal components. It is therefore important to control and to limit these deposits as much as possible by using additives which are effective and stable at high temperatures.
The protection of lubricating oils by means of antioxidant organic molecules is widely described in the prior art. A distinction is generally made among these compounds according to their mode of action: on the one hand, free radical inhibitors (hindered phenols, aromatic amines, phenothiazines, etc) and, on the other hand, products which destroy peroxides (metal dithiophosphates or dithiocarbamates).
There are also known metal deactivators such as, for example, mercaptobenzothiazole, which protect the lubricants from the catalytic oxidation of metal ions such as cuprous or cupric ions. These compounds act by passivating the surfaces or by chelating the metals present in the oil.
The use of antioxidant molecules of low molecular weight can present a number of disadvantages; in particular, the solubility of these products in oil is limited and they are sometimes volatile or capable of subliming when the motor operates in conditions close to its maximum output.
These disadvantages can be eliminated by employing viscosity-improving polymers which carry antioxidant functional groups. Another advantage of these multifunctional polymers is that they concentrate the antioxidant structures carried by the polymer in the upper parts of the engine during "flash" evaporations of the oil, where the protection against oxidation is most needed.
Viscosity-improving and antioxidant polymers are known in the prior art.
U.S. Pat. No. 4,500,440 claims an ethylene-propylene copolymer carrying epoxide functional groups obtained by grafting glycidyl methacrylate and treated with 5-aminotetrazole.
European Patent Applications 199,453 and 20,164 describe the introduction of molecules with an antioxidant effect, such as phenothiazine, or Schiff bases formed from aldehydes and nitrogenous heterocyclic rings such as pyrrolidine, morpholine or piperazine with ethylene-propylene copolymers. Both these patents describe processes for the preparation of antioxidant polymers involving a stage of grafting the antioxidant monomer in the presence of free radical initiators.
This process exhibits two major disadvantages: on the one hand, the antioxidant monomer can, on account of its functional group, react with peroxides and, on the other hand, can form homopolymers. Both these secondary reactions give rise to the formation of byproducts which cannot be employed and are difficult to separate.
European Patent Application 274,589 describes the preparation of lubricating compositions by condensing mixtures of phenothiazine and N-methylpiperazine with copolymers of methacrylic esters and of maleic anhydride. This synthesis is limited by the low reactivity of phenothiazine. A large quantity of unreacted phenothiazine remains dispersed in the polymer concentrate and can subsequently produce deposits in the engine.